The present invention relates to apparatus for generating a magnetic field having a spatially predetermined field pattern in a useful volume which is provided with bodies of ferro-magnetic material influencing the field pattern. Such apparatus are known, for instance, from DE-OS No. 25 26 845.
In apparatus by which magnetic fields can be generated, a spatially predetermined field pattern in a useful volume must frequently be adhered to with only small deviations. This applies, for instance, to particle accelerators in which deflection devices for charged particles such as electrons have suitably curved dipole magnets due to their curved particle tracks (see, for instance, "IEEE Transactions on Nuclear Science", Vol. NS-30, No. 4, Aug. 1983, pages 2531 to 2533). The predetermined field pattern is generated generally by a suitable shape and design of the current-carrying windings or also by ferro-magnetic pole pieces.
In the case of low magnetic field intensities or high field change rates, a number of interference sources distorting the field can become important so that then the field error limits to be maintained may be exceeded. Thus, one must think of external field disturbances such as the field of the Earth or magnetized objects as the cause of undesired field distortions. In addition, also eddy currents in metallic parts of the magnet itself or in the conductor can lead to such disturbances. Also, superconducting shielding currents in the filaments of a superconducting winding or the residual magnetization in an iron yoke represent such sources of disturbances. Finally, also the field of magnetizable, i.e., para-, ferri- or ferro-magnetic parts of magnetic apparatus can be the cause of field distortions.
In order to compensate for such field distortions, for instance, current-fed compensation windings can be provided which are frequently attached as a set of cylindrical multipole coils about the predetermined useful volume. These coils are fed by power supplies such that the previously measured field error is compensated in operation. Thus, for instance, a sextuple correction coil with a superconducting deflection magnet is known from the publication "Proc. 1972 Applied Supercond. Conf." Annapolis, U.S.A., pages 293 to 299.
Compensation of field distortion in a superconducting short-circuited multipole coil is also provided in the publication "Proc. 8th Int. Conf. on High-Energy Accelerators, CERN 1971", Geneva, Switzerland, 1971, pages 177 to 182. For this purpose, the undesired multipole error automatically induces, when running up the magnetic field, the coil current which is required for a compensation coil and then largely compensates this component in the useful volume. However, a separate coil is required for each multipole.
From DE-OS No. 25 26 845, magnetic apparatus for generating inhomogeneous magnetic fields is known which can be used, for instance, for magnetic ore separators. This magnetic apparatus has superconducting magnet coils in order to bring about the forces, depending on the product B grad B, on the particles to be separated. In order to generate a product B grad B as large as possible, bodies of ferro-magnetic material with higher field strength are provided in the known device.